The present invention pertains to a method for partially coating a cylindrical carrier body with a coating suspension. The invention pertains, in particular, to a method for coating carrier bodies for catalystsxe2x80x94for example, automobile exhaust gas catalysts.
Carrier bodies for automobile exhaust gas catalysts have a cylindrical shape with two end faces and an exterior surface jacket, and are provided with a series of flow channels for the exhaust gases of the internal combustion engine which lie parallel to the cylindrical axis extending from the first end face to the second end face. These carrier bodies are also referred to as honeycomb bodies.
The cross-sectional shape of the carrier body depends on the installation requirements of the motor vehicle. Carrier bodies with round, elliptical or triangular cross sections are broadly utilized. The flow channels usually have a square cross section and are arranged tightly adjacent to one another over the entire cross section of the carrier body. Depending on the type of application, the channel or cell density of the flow channels varies between 10 and 120 cmxe2x88x922. Honeycomb bodies with cell densities up to 250 cmxe2x88x922 and more are being developed.
Catalyst carrier bodies obtained by extruding ceramic masses are primarily used for purifying automobile exhaust gases. Alternatively, catalyst carrier bodies consisting of metal foils that are corrugated and subsequently wound are also available. Today, ceramic carrier bodies with cell densities of 62 cmxe2x88x922 are still predominantly used for purifying exhaust gases of passenger cars. The cross-sectional dimensions of the flow channels are 1.27xc3x971.27 mm2 in this case. The wall thicknesses of such carrier bodies lie between 0.1 and 0.2 mm.
Dispersed metals of the platinum group, the catalytic effect of which may be altered by compounds of base metals, are most frequently utilized for converting the harmful substances contained in automobile exhaust gases, e.g., carbon monoxide, hydrocarbons and nitrogen oxides, into harmless compounds. These catalytically active components need to be deposited on the carrier bodies. However, it is impossible to ensure the required dispersion of the catalytically active components on the geometric surfaces of the carrier body by depositing these components. This applies to non-porous metallic carrier bodies as well as porous ceramic carrier bodies. A sufficiently large surface for the catalytically active components can only be provided by applying a support layer of fine-particle, high surface area materials onto the inner surfaces of the flow channels. This process is referred to in the following as the coating of the carrier body. A coating of the outer envelope, or exterior surface jacket, of the carrier body is undesirable and should be prevented in order to avoid losses of valuable catalytically active materials.
A suspension of the fine-particle, high surface area materials in a liquid phase, usually water, serves for coating the carrier bodies. Various methods for depositing the support layer on the carrier body, by utilizing the coating suspension or slurry, are known from the state of the art. The coating is, for example, realized by immersing the carrier body in the coating suspension or by pouring the coating suspension over the carrier body. It is also possible to pump or attract the coating suspension by suction into the channels of the carrier body. Excess coating material always needs to be removed from the channels of the carrier body by means of suction or compressed air. This also ensures that channels which might have become clogged with coating suspension are opened.
After the coating process, the carrier body and the support layer are dried and subsequently calcined, in order to solidify and fix the support layer on the carrier body. Subsequently, catalytically active components are introduced into the coating by means of an impregnation with usually aqueous solutions of precursor compounds of the catalytically active components. Alternatively, the catalytically active components can be added into the coating suspension. In this case, a subsequent impregnation of the finished support layer with catalytically active components is not necessary.
One essential criterion of the coating method is that the coating or charging concentration can be achieved in one cycle. This refers to the portion of solids which remains on the carrier body after the drying and calcining processes. The coating concentration is expressed in grams per liter of the volume of the carrier body (g/L). In practical applications, coating concentrations up to 300 g/L are required for automobile exhaust gas catalysts. If this quantity cannot be applied in one cycle with the respectively utilized method, the coating process must be repeated after drying and, if applicable, calcining the carrier body until the desired concentration is reached.
DE 40 40 150 C2 describes a method in which catalyst carrier bodies having a honeycomb shape can be uniformly coated with a support layer, or with a catalytically active layer, over their entire length. In the following description, catalyst carrier bodies with a honeycomb shape are also referred to as honeycomb bodies. According to the method described in DE 40 40 150 C2, the cylinder axis of the honeycomb body is vertically aligned for the coating process. Subsequently, the coating suspension is pumped into channels through the lower end face of the honeycomb body until it emerges at the upper end face. The coating suspension is then pumped off downward and excess coating suspension is removed from the channels by means of suction or compressed air in order to prevent clogging of the channels. Support layers that have an adequate uniformity over the entire length of the honeycomb body can be obtained with this method.
U.S. Pat. No. 4,550,034 and U.S. Pat. No. 4,609,563 describe a method for coating ceramic honeycomb bodies in which a predetermined quantity of a coating suspension is filled into a flat container and the honeycomb body to be coated is immersed into the suspension with one of its end faces. The predetermined quantity of the coating suspension corresponds to the desired coating quantity for the honeycomb body. Subsequently, the entire quantity of the coating suspension is attracted by suction into the flow channels of the honeycomb body by applying a vacuum to the second end face. Since the predetermined quantity of the coating suspension corresponds to the coating quantity required for the honeycomb body, no removal of excess coating suspension from the flow channels takes place after the coating suspension is introduced into the flow channels. The coating process is preferably carried out in two steps, with 50-85% of the required coating quantity being attracted by suction from the first end face in the first step, and with the remaining coating quantity being attracted by suction into the flow channels from the second end face of the honeycomb body.
A high reproducibility of the coating concentration can be achieved with the method described in these two patents. However, the thickness of the coating along the honeycomb body has a significant gradient in the catalysts manufactured in this manner. Also, the preferred coating of the honeycomb body in two steps is unable to sufficiently improve the uniformity of the coating along the honeycomb body.
Certain applications require catalysts that have regions with different catalytic activities along the catalyst carrier body. For example, EP 0 410 440 B1 describes a catalyst that consists of two partial catalystsxe2x80x94namely a catalyst on the inflow side which serves for achieving a selective catalytic reduction of nitrogen oxides by means of ammonia or a compound that supplies ammonia and an oxidation catalyst on the outflow side. In this case, the oxidation catalyst is applied in the form of a coating onto a section of the one-piece reduction catalyst that is fully extruded in a honeycomb shape, which section is situated on the outflow side, with the section on the outflow side amounting to 20-50% of the total catalyst volume. The application of the oxidation catalyst is realized by immersing the outflow side of the honeycomb body into the coating suspension for the oxidation catalyst, up to the desired length.
DE 195 47 597 C2 and DE 195 47 599 C2 describe the reinforcement of the end faces of monolithic catalysts for purifying exhaust gases by applying or introducing inorganic materials that reinforce the mechanical properties of the carrier body or the catalytic coating. The length of the reinforced zone may, measured from the respective end face, amount up to twenty times that of the channel diameter. In order to carry out this coating process, it is proposed to immerse the catalyst body into a suspension of the reinforcing materials or to spray this suspension onto the end faces of the body.
These examples show that there is a significant demand for coating methods for partially coating honeycomb bodies or carrier bodies. U.S. Pat. No. 5,866,210 describes such a method. The coating is realized by immersing one end face of the substrate into a bath containing the coating suspension. This bath contains an excess quantity of the coating suspension in comparison to the quantity required for coating the substrate up to a desired height. A vacuum is then applied to the second end face, with the intensity and duration of this vacuum sufficing for attracting the coating suspension into the channels by suction up to the desired height. In this case, it is attempted to achieve the most uniform coating height possible in all channels.
In the following, the transition between the freshly coated region of the carrier body and the remaining region of the carrier body is referred to as the coating edge.
The method according to U.S. Pat. No. 5,866,210 has several significant disadvantages. The height of the coating or its axial length is determined by the utilization of capillary forces as well as the intensity of the applied vacuum and the duration during which the vacuum is applied to the second end face of the carrier body. Values of 1-3 seconds are indicated for this duration. Changes in the viscosity of the coating suspension consequently lead to direct changes in the length of the applied coating, i.e., to an inferior reproducibility of the coating edge. The intensity of the vacuum is indicated in this U.S. patent as no more than 1 inch water column which corresponds to approximately 2.5 mbar. The precise control of this slight vacuum is also complicated and can result in additional problems regarding the reproducibility of the coating method. Due to the slight vacuum, only coating suspensions with a low viscosity can be processed with this method. This means that the suspensions used usually have a low solids content. This low solids content, in turn, requires that several coating processes be carried out successively in order to achieve a high coating concentration.
In the method according to U.S. Pat. No. 5,866,210, the capillary forces are very important. This means that this method is dependent upon the cell density of the carrier body to be coated.
After the coating suspension has reached the desired height in the flow channels, the carrier body is lifted at the second end face while the vacuum is preserved such that the contact with the coating suspension is interrupted. Due to the continued vacuum on the second end face, air is conveyed through the flow channels and the coating is at least partially dried. During this phase of the coating process, the vacuum is increased to 5-15 inch water column and maintained for an additional 2-4 seconds. Due to this measure, the coating edge may become smeared.
The present invention relates to a method for partially coating carrier bodies which makes it possible to use coating suspensions with a high solids content, and to achieve a high reproducibility of the position of the coating edge in the channels, with the reproducibility being largely independent of the cell density of the carrier body.
The above and other objectives of the invention can be attained with a method for partially coating a cylindrical carrier body with a coating suspension, in which the carrier body has a cylindrical axis, two end faces, an exterior surface jacket, and an axial length L, and a series of channels extending from the first end face to the second end face. A desired coating quantity of the coating suspension is applied onto the carrier body by vertically aligning the cylindrical axis and introducing the coating suspension through the lower end face. It is a feature of the method that the carrier body, measured from the lower end face, is filled with a filling volume of the coating suspension which is metered such that the carrier body is only filled up to a height that amounts to a predetermined fraction of its length L, and by the fact that excess coating suspension is removed downward such that the desired coating quantity remains on the carrier body.
It is essential for the method according to the invention that a defined filling volume of the coating suspension be introduced into the channels of the carrier body. Consequently, the filling volume corresponds to the empty volume of the channels up to the desired coating height. In the present method, the coating height consequently is adjusted by means of an exact volumetric metering of the coating suspension, with the coating height merely being determined indirectly in the form of a cooperation between several variables, e.g., capillary forces, vacuum and viscosity, in the methods known from the state of the art.
When carrying out this method, the filling volume needs to be increased by additional volume portions, namely the volume portions of required pipelines and, if applicable, other parts of the apparatus required for carrying out the method.
The filling quantity introduced into the channels of the carrier body is obtained by multiplying the filling volume of the coating suspension with the density of the coating suspension. One needs to differentiate between the filling quantity, the volume of which determines the coating height, and the desired coating quantity, which represents the coating quantity after excess coating suspension is removed from the channels of the carrier body. The quantity of the coating suspension which is introduced into the carrier body consequently is always larger than the coating quantity. The coating is dried and, if applicable, calcined after the excess coating suspension has been removed.